This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.

The pthread_cleanup_pop() function shall remove the routine at the
top of the calling thread's cancellation cleanup stack and optionally
invoke it (if execute is non-zero).
The pthread_cleanup_push() function shall push the specified
cancellation cleanup handler routine onto the calling thread's
cancellation cleanup stack. The cancellation cleanup handler shall be
popped from the cancellation cleanup stack and invoked with the
argument arg when:
* The thread exits (that is, calls pthread_exit()).
* The thread acts upon a cancellation request.
* The thread calls pthread_cleanup_pop() with a non-zero execute
argument.
These functions may be implemented as macros. The application shall
ensure that they appear as statements, and in pairs within the same
lexical scope (that is, the pthread_cleanup_push() macro may be
thought to expand to a token list whose first token is '{' with
pthread_cleanup_pop() expanding to a token list whose last token is
the corresponding '}').
The effect of calling longjmp() or siglongjmp() is undefined if there
have been any calls to pthread_cleanup_push() or
pthread_cleanup_pop() made without the matching call since the jump
buffer was filled. The effect of calling longjmp() or siglongjmp()
from inside a cancellation cleanup handler is also undefined unless
the jump buffer was also filled in the cancellation cleanup handler.
The effect of the use of return, break, continue, and goto to
prematurely leave a code block described by a pair of
pthread_cleanup_push() and pthread_cleanup_pop() functions calls is
undefined.

The restriction that the two routines that push and pop cancellation
cleanup handlers, pthread_cleanup_push() and pthread_cleanup_pop(),
have to appear in the same lexical scope allows for efficient macro
or compiler implementations and efficient storage management. A
sample implementation of these routines as macros might look like
this:
#define pthread_cleanup_push(rtn,arg) { \struct _pthread_handler_rec __cleanup_handler, **__head; \__cleanup_handler.rtn = rtn; \__cleanup_handler.arg = arg; \(void) pthread_getspecific(_pthread_handler_key, &__head); \__cleanup_handler.next = *__head; \*__head = &__cleanup_handler;
#define pthread_cleanup_pop(ex) \
*__head = __cleanup_handler.next; \
if (ex) (*__cleanup_handler.rtn)(__cleanup_handler.arg); \
}
A more ambitious implementation of these routines might do even
better by allowing the compiler to note that the cancellation cleanup
handler is a constant and can be expanded inline.
This volume of POSIX.1‐2008 currently leaves unspecified the effect
of calling longjmp() from a signal handler executing in a POSIX
System Interfaces function. If an implementation wants to allow this
and give the programmer reasonable behavior, the longjmp() function
has to call all cancellation cleanup handlers that have been pushed
but not popped since the time setjmp() was called.
Consider a multi-threaded function called by a thread that uses
signals. If a signal were delivered to a signal handler during the
operation of qsort() and that handler were to call longjmp() (which,
in turn, did not call the cancellation cleanup handlers) the helper
threads created by the qsort() function would not be canceled.
Instead, they would continue to execute and write into the argument
array even though the array might have been popped off the stack.
Note that the specified cleanup handling mechanism is especially tied
to the C language and, while the requirement for a uniform mechanism
for expressing cleanup is language-independent, the mechanism used in
other languages may be quite different. In addition, this mechanism
is really only necessary due to the lack of a real exception
mechanism in the C language, which would be the ideal solution.
There is no notion of a cancellation cleanup-safe function. If an
application has no cancellation points in its signal handlers, blocks
any signal whose handler may have cancellation points while calling
async-unsafe functions, or disables cancellation while calling async-
unsafe functions, all functions may be safely called from
cancellation cleanup routines.

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 PTHREAD_CLEANUP_POP(3P)